The present invention is aimed at resolving certain problems inherent in single-cylinder two-stroke engines with a valve intake into the crankcase via a carburetor and intake pipe, the engine elements being between two planes which are relatively close together and perpendicular to the engine crankshaft axis.
Such motors are found in particular in motorcycles wherein a magnetic flywheel on the one hand and the transmission elements which may comprise, for example, clutched and a speed governor, on the other hand, are mounted on the crankshaft axis on both sides of the cylinder. The engine is thus contained between two planes perpendicular to this axis, it being impossible to move the two planes apart at will for obvious space reasons.
On the other hand, to permit accessibility, the carburetor is generally mounted in such a motorcycle with its flow axis essentially parallel to the crankshaft axis, but the space occupied by this carburetor has meant that that far a flat valve has been used for admission of gases into the crankcase. It is known that such a valve has a plate in which an orifice is provided which can be blocked or cleared by means of a flexible blade. This valve, placed in the gas stream, has the disadvantage of causing turbulent flow and hence load losses, even if the flow-pipe axis is inclined with respect to the direction perpendicular to the plate.
Finally, in known engines of the above type the valve axis and the downstream part of the intake pipe which connects the carburetor outlet to the valve inlet are located in a plane perpendicular to the engine's crankshaft axis.